Although we've reached a time when we can identify exoplanets, we have yet to find one that is habitable.

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Los Angeles, CA - Earlier this month, an alien world was discovered orbiting a distant star. This world - known as an "extrasolar planet" or simply an "exoplanet" - is over eight times the size of the planet Jupiter and is zipping around its host star (called "KOI-13") extremely fast. One "year" (or one orbit) for this world - called KOI-13b - lasts only 42 hours. It is therefore orbiting its star very closely; KOI-13b is very hot, the hottest exoplanet discovered to date. In fact, this exoplanet is hotter than some of the coolest stars!

KOI-13b may sound exotic but, at time of writing, its discovery is only the latest of 759 confirmed exoplanetary finds in recent years. Though many of these discoveries are even more exotic than

Although there are over two thousand "candidate" exoplanets, Kepler must detect four "transits" - when a planet passes in front of a star - in order to confirm its existence [GALLO/GETTY]

Los Angeles, CA - Earlier this month, an alien world was discovered orbiting a distant star. This world - known as an "extrasolar planet" or simply an "exoplanet" - is over eight times the size of the planet Jupiter and is zipping around its host star (called "KOI-13") extremely fast. One "year" (or one orbit) for this world - called KOI-13b - lasts only 42 hours. It is therefore orbiting its star very closely; KOI-13b is very hot, the hottest exoplanet discovered to date. In fact, this exoplanet is hotter than some of the coolest stars!

KOI-13b may sound exotic but, at time of writing, its discovery is only the latest of 759 confirmed exoplanetary finds in recent years. Though many of these discoveries are even more exotic than KOI-13b, there are many other worlds that have a decidedly "Earth-like" flavour.

So when will a bona fide "second Earth" be found? And if we do find Earth's twin, what significance will it have? Why are we even looking for these exoplanets in the first place? Unfortunately, although we are currently undergoing a revolution of scientific discovery, the spectre of science budget cuts may prevent these profound questions from finding an answer.

Hunting for Earth's twin

In 2009, NASA launched the Kepler space telescope to begin the search for small worlds orbiting distant stars. Named after Johannes Kepler - the 17th century astronomer who deciphered the orbits of the planets in the Solar System, thereby heralding the infancy of modern astronomy - the space telescope is continuously staring at a small patch of sky containing around 170,000 stars in the Milky Way.

The Kepler space telescope works by detecting exoplanets as they pass in front of their parent stars - an event known as a "transit". The telescope is so sensitive that it not only detects the very faint dimming of starlight as an exoplanet orbits in front of it, Kepler can also measure the exoplanet's physical size - the bigger the dimming event, the bigger the exoplanet. To date, the Kepler mission has identified over 2,300 candidate alien worlds, but its work is far from done. To confirm an exoplanet's existence, Kepler must detect four transits. Until this happens, there's some ambiguity as to whether the exoplanet candidate exists at all.

Although hundreds of exoplanets were discovered before the launch of Kepler, one of Kepler's key attributes is that it can detect very small worlds. In fact, Kepler has already found exoplanets approximating the size of Earth, and some of Kepler's candidates are as small as Mars! This is fortunate as astronomers have estimated that two-thirds of the stars in our galaxy may have Earth-mass worlds orbiting them.

Late last year, the discovery of two worlds orbiting a star called Kepler-20 made history - they became the first ever confirmed Earth-sized exoplanets to be discovered outside of our Solar System. The discovery of Kepler-20e and Kepler-20f hit international headlines and it was beginning to sound as if Earth's twin (and cousin) had been discovered! Unfortunately, that news was premature. Apart from their size - Kepler-20e is a bit smaller than Venus and Kepler-20f is only 3 per cent wider than Earth - there is nothing"Earth-like" about the pair. Like KOI-13b, they orbit very close to their star, making them hellishly hot worlds where life "as we know it" wouldn't survive.

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So, we are discovering "Earth-sized" (not "Earth-like") worlds; when will we find a world that is Earth-sized as well as being heated to just the right temperature to support life as we know it?

The "Goldilocks Zone"

One of the key objectives for astronomers looking for "Earth-like" worlds is to find exoplanets that have the potential to support life. The only precedent for life in our universe is on our planet - we haven't seen any extraterrestrials (yet), so the only habitat we know that life is supported is on Earth. But what makes Earth so special?

For starters, our planet has a nitrogen-rich atmosphere. Secondly, it orbits a star (the Sun) at just the right distance to maintain liquid water on its surface. If Earth orbited the Sun a little closer, all our oceans would boil off; any further away and all water would freeze. Liquid water maintains all life on our planet, so if the water was constantly in a vapor (steam) or solid state (ice), life would not have evolved. Earth therefore exists within a very special region around the Sun that's not too hot and not too cold, it's "just right". This is commonly known as the "Goldilocks Zone" or, more scientifically, the "Habitable Zone".

Kepler has the ability to measure the physical size of a transiting exoplanet and it can measure the orbital distance from its star. Using a quick calculation, the Kepler science team can therefore decipher whether or not a candidate exoplanet orbits within its star's habitable zone. Unfortunately, Kepler cannot detect whether or not an exoplanet has an atmosphere.

So, has Kepler spotted an exoplanet orbiting its star within the habitable zone yet? Yes.

On December 5, 2011, the Kepler science team announced another historic discovery: an exoplanet orbiting a Sun-like star smack bang in the middle of its habitable zone. It has an orbital period approximating a year on Earth - 290 days. If there's water on its surface, it should be in a liquid state, and therefore logic dictates that life may be possible. But there's a catch. The exoplanet, called Kepler-22b, is known as a "super-Earth" as it is substantially larger than our planet. Kepler-22b is 2.4 times bigger than Earth.

Although Kepler-22b's size doesn't automatically disqualify it from having life-supporting qualities, the Kepler mission cannot see if this super-Earth even has a rocky "surface" - it could be a small gaseous planet like a mini-Neptune. As far as we know, Neptune isn't the kind of place life would like to evolve.

Tantalising clues

Earth's twin discovered beyond solar system

It seems that every month, huge exoplanetary discoveries are being made; we are currently experiencing a historic time in human history. Astronomers are using a space telescope to stare at a patch of sky that isn't particularly special, and yet they have already detected hints of thousands of alien worlds. Only a few short years ago, we had no idea whether planets even existed beyond the Solar System (although we had a pretty good idea that they did). But Kepler has completely revolutionised our perspective on the universe: planets appear to be as common as the stars we see in the night sky.

As more exoplanets are counted and more alien worlds roughly the size of Earth are discovered, we are gradually getting closer to the discovery of an Earth-sized world, orbiting a Sun-like star within its habitable zone. But even then, Kepler cannot tell us whether that world will have an atmosphere or whether it has the ability to support life. Bigger and better telescopes need to be constructed to build on Kepler's groundbreaking discoveries.

Unfortunately, despite Kepler's amazing track record, its primary mission is drawing to a close. After 3.5 years of operations, mission funding is set to end, and so far there is no promise of a mission extension. Ironically, the initial Kepler mission was tailored in the hope of detecting a true Earth-sized world orbiting a Sun-like star within the habitable zone. 3.5 years is the minimum time that a world like this may be confirmed to exist - remember, Kepler needs 4 orbits for an exoplanetary discovery, the Kepler science team therefore needs to wait 3.5 Earth years for such a discovery to happen. They came close with the discovery of super-Earth Kepler-22b, but not close enough.

By the end of this year, we may be close to the historic announcement of the discovery of Earth's twin, but NASA's budget is looking grim and a mission extension may not be on the cards. Only time will tell if the politics of science spending will put an abrupt end to this revolutionary age of exoplanetary discoveries.

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